How long does it take for illness to spread to other chickens??

On Jan 29th I got 3 new pet chickens, younger than the 5 other chickens I have. After I let the chickens out of their coop this Monday I heard one of the older ones making noise while breathing, even more so as I chased her around to catch her, the right side of her face was also a tiny bit swollen so I seperated her in my room from the rest of my flock. The next day as i checked up on my other chickens another older one, my bantam, starting making similar noises when breathing, i tried keeping her away from the rest as well but then that same night I heard my Australorp (again one of my older chickens) making the noise as well. So now i have 3 chickens making a "gargling" noise when breathing, bantam having more of a runny nose while the other two look mostly clean, all sneezing. I noticed they make the most gargly noise when i handle them. Unfortunately I have no more room to quarantine the big Australorp, she's outside with the others while i have the bantam and claret in my room.

Is it possible that my older chickens got anything from the new ones even if they don't have the same symptoms other than one of them have had a slight runny nose, no gargling? I noticed her runny nose a day or two after getting her, now for the most part her beak looks clean. Did this come from something else? How quick do chickens catch something from a new bird if it's sick?

I took the claret to a vet yesterday and we're trying antibiotics that they gave me for a week. Getting those 3 new chickens and switching to starter/grower feed is the only drastic changes I've done with my flock so far.

I also have an aviary outside that houses doves and a few other birds, is it possible that whatever my chickens are having can spread onto them? They free range all over my backyard.

Yes, your birds likely did get something from the ones you brought it. Sounds possibly like mycoplasma from the sounds of it. Its incubation period is one to three weeks, so it's not surprising you're just seeing symptoms now. Yes, the disease can spread to your other birds.

The new hens were likely asymptomatic carriers and they have passed the illness to the rest of your flock. That's why quarantine is so important; even seemingly healthy birds can be harboring nasty things.

It's probably best that you find out for sure what this is if possible so you can decide how to move forward. There are tests you can have your vet do on a live bird; they are upwards of $100 though. I can send you a link to a site that will do this testing too if you want. The other option is to cull a sick one and send it off for a necropsy.

Until you know what it is, assume the worst so that you don't infect others' flocks. Run a closed flock - no bird ever leaves your property and no eggs leave for hatching since mycoplasma can pass through the egg and infect chicks before they even hatch.

If it is a disease such as mycoplasma or coryza, your birds will now carry it for life, pass it to all other birds they come into contact with, and will have flare ups whenever they are stressed.

There are a few viral things it could be too, such as infectious bronchitis, and they are pretty nasty as well. As I said, it would be best if you could find out what you're dealing with here.

Good advice in above post. Also the wild birds poo can pass disease to your birds . As far as not having the same symptoms , it still could be the same virus and the older birds are handling different . Sadly if it is a viral infection antibotics will NOT kill the virus. However they CAN help with secondary infections that often come along with the virus. Also , once the symptoms of the virus pass your birds can still carry the virus and infect other birds and chickens. Its important to know what the virus is if possible this way you might be able to protect the newer chickens with a vaccine if you wish. However certain live virus vaccines can also shed virus to others.